Two-conductor bidirectional digital seismic telemetry interface

ABSTRACT

A two-conductor bidirectional digital telemetry interface between a seismic sensor acquisition/conversion module and a seismic data collection module. The data collection module is configured as a master electronics device and the sensor acquisition/conversion module is configured as a slave electronics device in the telemetry system. The master device provides power to the slave device over the two conductors. The master device transmits portions of commands to the slave device at a first time and the slave device transmits portions of a digital seismic data packet to the master at a different second time in a fixed-duration frame. The frames are transmitted at regular intervals. The outbound commands and inbound data are encoded by block codes. A phase-locked loop in the slave is locked to a master clock in the master by deriving a clock and a sync point from the block-coded commands it receives from the master. The block code representing each command bit minimizes dc drift and provides a level transition in the command that can be used to maintain synchronism between master and slave.

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional ApplicationSerial No. 60/175,500, filed Jan. 11, 2000.

TECHNICAL FIELD

The invention relates generally to seismic prospecting and, morespecifically, to telemetry between a master electronics module, such asa data collector, and one or more slave electronics modules, such asseismic sensor data acquisition modules.

BACKGROUND

Conventional geophones and hydrophones used in seismic prospecting eachhave a dedicated two-wire connection to conduct analog seismic signalsto acquisition/conversion circuitry. The analog signals from one or moreremote seismic sensors (hydrophones, geophones, or other seismicsensors) are sampled and converted to a series of digital values by theacquisition/conversion circuitry. The acquisition/conversion circuitryis typically configurable to, for example, adjust the sampling rate,alter any digital filtering or other digital signal processingparameters, or perform diagnostics.

One or more of these acquisition/conversion circuits are connected to adata collection unit. Each data collection unit collects the series ofdigital values for all the seismic sensors connected to all theacquisition/conversion units connected to it. The data collection unitpasses that data to a seismic recording system, including a systemcontroller, over a high-speed data link, such as a fiber-optic cable.

Conventionally, however, the digital interface between anacquisition/conversion unit and a data collection unit has comprised atleast four wires in two pairs: two wires (one pair) used for a digitalcommand signal to the acquisition/conversion unit and two wires (theother pair) for the digital seismic data from the acquisition/conversionunit. In addition, power is supplied to the acquisition/conversion unitover the “phantom pair” formed by the two pairs of telemetry wires orover separate dedicated power wires.

Although the conventional four-wire telemetry works, it does haveshortcomings. First, the weight of a cable depends on the number ofwires and the concomitant amount of copper it contains. Second, thediameter of a cable depends on the number and size of wires it encases.Third, more wires require more connections to be made, which increasesthe chances of incorrect or unreliable connections.

It should be clear that there is a need for a smaller, lightweight,standard physical interface that can send commands and deliver powerfrom a data collector to one or more remote sensor acquisitionconversion units and transmit seismic data from the sensoracquisition/conversion units to a data collector.

SUMMARY

The shortcomings of conventional seismic telemetry systems are overcomeand the needs satisfied by a two-conductor bidirectional digital seismicinterface having features of the invention. The interface comprises atwo-conductor line connected between a master electronics module, suchas a data collection module, and a slave electronics module, such as aseismic sensor electronics module. Digital commands are transmitted fromthe master electronics module to the slave sensor electronics module inone direction along the two-conductor line. Digital data from the slavesensor module are transmitted back to the master module in the oppositedirection over the same pair of conductors to form a bidirectionalinterface. The slave module includes a phase-locked loop that derivesclock information from the outbound command signal to keep the looplocked for coherent data acquisition and to derive a synchronization, orsync, point for properly decoding commands issued by the master. Blockcodes used by the master to encode the command bits that constitute agiven command are selected to guarantee a level transition coincidentwith the sync point in the slave and to minimize dc drift. In this way,the slave remains synchronized with the master to enable synchronizedbidirectional telemetry.

In a preferred version of the telemetry interface, the master transmitsa synchronization pattern to lock the phase-locked loops in the slavesand to establish the sync point. Portions of outbound command bits andinbound data are confined to individual fixed-duration frames. Completecommands and data are apportioned among consecutive frames.

The slaves are preferably powered by a dc power supply at the masterconnected across the two-conductor pair. In this way, only twoconductors are needed to handle bidirectional telemetry and to supplypower, instead of the conventional four or more. This allows digitalsensors to be used in place of analog sensors with only minormodifications of master and slave electronics.

BRIEF DESCRIPTION OF THE DRAWINGS

These features, advantages, and aspects of the invention are betterunderstood by reference to the following description, appended claims,and accompanying drawings in which:

FIG. 1 is a schematic block diagram of a two-conductor telemetryinterface embodying features of the invention;

FIG. 2 is a timing diagram representing a fixed-duration timing frameused in the telemetry system of FIG. 1;

FIG. 3 is a timing diagram illustrating the relationship between atraining signal and a command signal in a telemetry system as in FIG. 1;

FIG. 4 is a timing diagram showing early and late gates used to controlthe voltage-controlled crystal oscillator (VCXO) in a slave phase-lockedloop circuit in the telemetry system of FIG. 1;

FIG. 5 is a diagram of the return data field format used in thetelemetry system of FIG. 1; and

FIG. 6 is a block diagram of another version of the two-conductortelemetry interface of FIG. 1 including repeaters to permit theinterconnection of a series of slave electronics modules on a singletelemetry channel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A specific example of the two-conductor telemetry interface embodyingfeatures of the invention is shown in the schematic block diagram ofFIG. 1. A master electronics module 20 includes data collectioncircuitry for collecting data from a number of remote sensors 22, whichmay be geophones, hydrophones, or other seismic-sensitive devices usedin seismic prospecting—land or marine. The seismic data from the sensorsare collected and transmitted over a high-speed data link 24, such as afiber-optic cable, to a central recording system 25. The centralrecording system includes a main controller configuring the seismic datacollection system and issuing supervisory commands to the master moduleover the high-speed link. Conventional electronic circuits and/orfirmware are used to implement the data collection and high-speedinterface indicated by block 26. The block 26 preferably includesdigital logic to format an outbound command on CMD OUT line 28 and otherdigital logic to receive formatted data bytes inbound on DATA IN line29. The data collector 26 combines the data from all the remote sensors,organizes the data, and transmits the data to the recording system overthe high-speed link.

The master module telemetry interface circuits include, in the outboundcommand path, an encoder 30 and a differential line transmitter 32. (Forthe purposes of this specification, the term “outbound” refers tosignals directed from the recording system toward the sensor; the term“inbound” refers to signals directed oppositely.) Control logic 34 inthe master module controls the bit timing of the outbound encoder 30 insynchronism with a stable and accurate master clock 35, which mayoptionally be realized with a phase-locked loop (PLL). The control logicalso controls the state of the transmitter 32, turning it on when acommand is to be transmitted outbound and turning it off otherwise.Encoded outbound commands are coupled onto a two-conductor line 36,preferably a twisted pair, through a transformer 37 and capacitors 38.The two-conductor line connects the master module to a slave electronicsmodule 40 that includes acquisition and conversion circuitry 42 tosample and digitize the analog seismic signals from the sensors 22.

The slave module is powered by a de power supply 44 in the master moduleover the two-conductor line, each conductor connected to a terminal ofthe power supply through high-frequency chokes 41. A dc/dc converter 46in the slave unit converts the dc voltage on the two-conductor line intothe dc voltage levels V_(S) required by the electronics in the slavemodule. Blocking capacitors 39 and high-frequency chokes 43 in the slavemodule isolate the dc power from the command and data signals in thesame way as the capacitors 38 and high-frequency chokes 41 in the mastermodule.

An outbound command is transmitted onto the two-conductor line andreceived by a differential line receiver 48 through the blockingcapacitors and a transformer 45. The command is decoded in a decoder 50to produce a command input signal (CMD IN) to the acquisition/conversioncircuit 42. The CMD IN signal is interpreted by theacquisition/conversion logic to configure the acquisition system,perform diagnostics, or request certain data, for example.

Control logic 52 in the slave derives a clock signal from the decoder 50to which a phase-locked loop (PLL) 54 locks itself. In this way, themaster and slave modules are able to synchronize to each other forreliable communications. The control logic also turns the receiver 48 onto receive outbound commands and off when seismic data are transmittedinbound.

The slave unit of FIG. 1 shows four sensors connected to it by two-wirelines 56A-D. Sensor outputs are converted to digital sample values byone or more A/D converters in the acquisition/conversion unit 42. Theacquisition/conversion unit formats the data into a message packet andsends it out as a digital signal (DATA OUT) containing a sequence ofdata in block-encoded Non-Return-to-Zero (NRZ) format to a data encoder58. Using the derived clock from the inbound command, the control logic52 turns on a differential line transmitter 60 to couple the encodeddigital seismic data onto the master-slave telemetry interface throughthe transformer 45 and capacitors 39. The control logic uses the timingclock produced by the locked PLL to generate an encoded inbound datasignal synchronized to the master's clock.

The inbound data signal travels over the two-conductor line 36, throughthe blocking capacitors 38 and transformer 37, to a differential linereceiver 62 in the master module. The master control logic 34 ensuresthat the receiver is turned on when inbound sensor data are expected.The received telemetry data are decoded in a data decoder 64, whosetiming is controlled by the master control logic 34. The digital signalDATA IN is decoded in the master data collection circuitry 26, whichthen transmits the packetized data on the high-speed link 24 to therecording system.

Another version of the two-conductor telemetry system is shown in FIG.6. Repeater circuits 57 in first 40′ and second 40″ slave electronicsmodules allow slaves to be serially connected on a single data channelto the master 20 by intervening two-conductor lines 36. The repeater ineach slave resynchronizes and retransmits the outbound and inboundsignals and ensures that the signal amplitudes are at a sufficientlevel. The repeater circuits derive timing information from the outboundcommand circuit to lock their PLLs to the master clock. Each immediateupstream slave looks like a master to its downstream neighbor. Bystringing a series of slaves with repeaters together, a single mastercan support multiple slave modules using two conductors.

In the two-conductor bidirectional telemetry of the invention, commandsare transmitted by the master 20 to the slave sensor module 40 in theoutbound direction, and sensor data are transmitted in the oppositeinbound direction over the same pair of conductors 36. Each command bitis encoded using a block code. The block code ensures that the signal isdc-balanced and that a level transition occurs at the sync time to helpthe PLL stay locked. The features of the block code used to achievethese objectives are an equal number of 0's and 1's in each code and a0-to-1 transition in the middle of the block code. (A 1-to-0 transitioncould be used equivalently.) As shown in FIG. 2, outbound commands 72and inbound data 74 are interleaved in a concatenation of individualcommand frames 70 in a time-division multiple-access (TDMA) scheme. Withthe master control logic 34 turning on the line transmitter 32 andturning off the line receiver 62, the command 72 is transmitted at afirst time at the start of the command frame 70. After a delay D₁,determined by the length of the line 36, the command 72′ is received bythe slave 40, whose line receiver 48 is on and whose transmitter 60 isoff as controlled by slave control logic 52. After the slave receivesthe command, the slave control logic turns off the receiver and turns onits transmitter. A wait interval W ensues, after which the return data74 are sent inbound at a second time to the master, which receives thedata after a delay D₂, wherein D₁=D₂=D.

In the preferred embodiment, each command frame 70 is 15.625 μs induration, or 64 bits long at a bit rate of 4.096 Mbps. Block codes areused to encode the command and return data. Each bit of a command isencoded as an eight-bit block code at a bit rate of 4.096 Mbps. Anexample set of block codes for the command bit are shown in Table I. A“0” bit is represented by the block code 01001101; a “1” bit isrepresented by 10001110. (Of course, other codes having the desiredfeatures are possible.) Usually, only a portion, i.e., one bit(represented by eight bits of block code), of a command sequence istransmitted at the start of each command frame. Thus, it takes a numberof consecutive command frames for the entirety of a command to betransmitted, except in the case of single block code commands ormarkers.

TABLE I Information Block Code Command Data bit = 0 01001101 CommandData bit = 1 10001110 Time Align Marker 01001110

The return data packet is encoded differently. During each commandframe, eight bits of the sequence of seismic packet data are transmittedinbound from the slave to the master. The eight data bits are dividedinto two four-bit portions. Each four-bit portion is encoded by aneight-bit block code as shown in the example of Table II. The bits ofthe block code are clocked at 4.096 Mbps. As shown in FIG. 5, the returndata in each command frame include two block-encoded eight-bit blocks76,76′ preceded by an eight-bit slave ID code 77. Thus, the return datafield in each command frame is 24 bits long. Unless all the data fit inthe 24-bit field, the return data, like the command, will be apportionedamong a number of consecutive command frames.

TABLE II Four Data Bits Block Code 0000 01001101 0001 01010110 001001011001 0011 01011010 0100 01100101 0101 01100110 0110 01101001 011101101010 1000 10010101 1001 10010110 1010 10011001 1011 10011010 110010100101 1101 10100110 1110 10101001 1111 10110010

Because only one command bit is encoded in each command frame, theeffective command data transmission rate is 64 kbps. The slave's controllogic derives a 64 kHz signal from the command transmission to keep thePLL in sync. Because eight data bits of the sequence of seismic packetdata (two four-bit blocks) are transmitted inbound in each commandframe, the effective return data transmission rate is 512 kbps.

From FIG. 2, the length of line that can be used with this telemetryscheme can be derived as 2D+W<[Frame−(Cmd+Data)], where all values arein bits, or 2D+W<64−(8+8+8+8), or 2D+W<32. Thus, as shown in Table III,collisions between outbound commands and inbound data are not a problemfor common station (master-to-slave) spacings.

TABLE III Cable Two Way Two Way Length Cable Length Time (2 * D) 55 mstation spacing (cable length) 1 station  55 m  550 ns 2.3 bits 2station 110 m 1100 ns 4.5 bits 3 station 165 m 1650 ns 6.7 bits 88 mstation spacing (cable length) .5 station  44 m  440 ns 1.8 bits 1station  88 m  880 ns 3.6 bits 2 station 176 m 1760 ns 7.2 bits 2.5station 220 m 2200 ns 9.0 bits 3 station 264 m 2640 ns 10.8 bits 

To lock the PLL 54 in the slave to the clock 35 in the master, themaster transmits a long training sequence whenever the slave loses lock.The training sequence 78, which is illustrated in FIG. 3, has thepattern “1100” for about 8 ms until the training sequence sync code“10101100” is inserted, followed by about two final cycles of thetraining sequence. After the training sequence is transmitted, thenormal command sequence begins. Each bit of the training sequence isclocked at a 4.096 MHz rate. This means that the training sequence is a1.024 MHz square wave until the sync code. The slave derives a syncpoint from the rising transition 80 in the middle of the timing sequencesync code.

A more detailed description of the use of the training sequence andwaking up the slave is as follows: First, the master turns on power tothe slave. After waiting for the slave to initialize itself, the masterturns on its transmitter and transmits the training signal. The slave,its receiver on after initialization, finds the training signal patternafter a number of cycles. The slave enables its PLL and waits for it tolock to the outbound training signal. Once its PLL is locked, the slavewaits for the sync signal (10101100) to set the start of the commandframe. The master then transmits subsequent command packets at 15.625 μsintervals after the sync signal. The master stops transmitting thetraining signal and starts transmitting command frames every 15.625 μs .The slave waits for the master to assign it a slave ID before the slavetransmits data fields back to the master. At this point, the slave is inthe operating mode, alternately receiving commands and transmittingdata. The interleaved transmission of commands to the slave and datapackets to the master as shown in FIG. 2 continues.

Each command bit starting each command frame is represented by aneight-bit block code. The eight bits straddle the PLL sync point asshown in FIG. 3. To keep the PLL locked, the command bits include alow-to-high transition in mid-block. The transition is present incommand blocks of “0” and “1.” The PLL is defined to be locked when thelow-to-high transition of the training signal or the command block of acommand frame has occurred at the sync point for a number of consecutivecommand frames. This lock detect is implemented, in a preferredembodiment, with a counter in the slave control logic 52 that isincremented by one each time the transition of the training signal orthe command block occurs within a lock window and is decremented by foureach time the transition is outside the window. The lock window is onecommand bit wide and centered on the PLL sync point 80. Once the counterhas reached its maximum value and the PLL is locked, it takes twoconsecutive command frames without a synchronized transition for the PLLto lose lock.

The PLL's voltage-controlled crystal oscillator (VCXO) controls thefrequency of the locked PLL. In FIG. 4, an early gate 82 signal and alate gate 83 signal in the slave control logic 52 are used to determineif the PLL frequency has shifted relative to the master clock. If thecommand block transition occurs during the early gate, the voltage tothe VCXO is adjusted to advance the phase of the PLL to get it back insync. If the command block transition occurs during the late gate, thevoltage to the VCXO is adjusted to retard the phase of the PLL to resyncit. As the PLL sync point is adjusted in the slave, the early and lategates are shifted accordingly.

The command field, or block, in each command frame consists of theeight-bit code of Table I. In addition to codes for logic “0” and “1,”there is another eight-bit code called a Time Align marker. The mastertransmits the Time Align marker once per data record or as required as atiming reference for all the slave sensor modules. The Time Align markeris used to realign timers in all of the modules to the same scan timingand is particularly useful in systems in which the slave electronicsmodules have free-running clocks rather than PLLs. Like the block codesfor “0” and “1,” the block code for the Time Align marker includes alow-to-high transition at mid-block and an equal number of 1's and 0's.

The return data from each slave is transmitted inbound to the master ina 24-bit data field as formatted in FIG. 5, for example. The data blockmay vary in size depending on the number of sensor channels, e.g., fourfor the arrangement of FIG. 1. An end block contains conventional packeterror detection code, such as a cyclical redundancy code (CRC) for theentire packet. The CRC could be eight or sixteen bits.

Thus, the telemetry interface described requires only a singletwo-conductor line to send commands and power to a remote sensor module,as well as to receive sensor data from the module.

Although the invention has been described in detail with respect to apreferred version, other versions are possible. For example, a mastermodule can support more than one telemetry channel. A separatetwo-conductor line can be dedicated to each slave channel. An individualduplicate differential line transmitter and receiver circuit andtransformer in the master electronics module would be dedicated to eachslave data channel. The block codes described are exemplary only; otherversions are possible. Delivery of power from the master to the slave isdescribed as being dc voltage, but can also be implemented using dccurrent, ac voltage, or ac current. The slave module can contain abattery to supply at least a portion of the electrical power to theslave module. Therefore, the spirit and scope of the claims should notbe limited to the description of the preferred version.

What is claimed is:
 1. A seismic telemetry interface, comprising: amaster electronics module; a slave electronics module; a two-conductorline connecting the master electronics module to the slave electronicsmodule; and wherein the master electronics module transmits at least aportion of a digital command to the slave electronics module over thetwo-conductor line at a first time and the slave electronics moduletransmits a portion of a digital data sequence to the master electronicsmodule over the two-conductor line at a different second time andwherein the master electronics module supplies at least a portion of theelectrical power to power the slave electronics module over thetwo-conductor line.
 2. A seismic telemetry interface as in claim 1wherein the master electronics module transmits consecutive portions ofa digital command at fixed intervals.
 3. A seismic telemetry interfaceas in claim 2 wherein the consecutive portions of a digital command areencoded to provide digital level transitions defining sync pointsoccurring at the fixed intervals.
 4. A seismic telemetry interface as inclaim 1 wherein the master electronics module transmits at least aportion of digital commands at fixed intervals defining contiguousframes, the first time marking the start of a frame and the second timeoccurring later in the frame.
 5. A seismic telemetry interface as inclaim 1 wherein the master electronics module includes a master clockand wherein the slave electronics module includes a phase-locked loop.6. A seismic telemetry interface as in claim 5 wherein the master clockin the master electronics module includes a phase-locked loop.
 7. Aseismic telemetry interface as in claim 6 wherein the slave electronicsmodule derives timing information from the digital command to lock thephase-locked loop in the slave electronics module to the master clock.8. A seismic telemetry interface as in claim 5 wherein the slaveelectronics module derives timing information from the digital commandto lock the phase-locked loop in the slave electronics module to themaster clock.
 9. A seismic telemetry interface as in claim 8 wherein thedigital command is encoded with digital level transitions spaced atfixed intervals to provide the timing information to lock thephase-locked loop in the slave electronics module.
 10. A seismic dataacquisition system, comprising a plurality of seismic sensors sendingseismic signals to the slave electronics modules and using the seismictelemetry interface of claim
 1. 11. A seismic telernetxy interface,comprising: a master electronics module, including a power supply; afirst slave electronics module; a second slave electronics module; afirst line composed of at most two conductors electricallyinterconnecting the master electronics module to the first slaveelectronics module; and a second line composed of at most two conductorselectrically interconnecting the first slave electronics module to thesecond slave electronics module; wherein the first slave electronicsmodule transmits at least a portion of a digital data command to thesecond slave electronics module over the second line at a first time andthe second slave electronics module transmits at least a portion of adigital data sequence to the first slave electronics module over thesecond line at a different second time, and wherein the power supply inthe master electronics module supplies at least a portion of theelectrical power to the first slave electronics module over the firstline.
 12. A seismic telemetry interface as in claim 11 wherein themaster electronics module includes a master clock and wherein the firstand second slave electronics modules each include a phase-locked loop.13. A seismic telemetry interface as in claim 12 wherein the masterclock in the master electronics module further includes a phase-lockedloop.
 14. A seismic telemetry interface as in claim 12 wherein thesecond slave electronics module derives timing information from thedigital command from the first slave electronics module to lock thephase-locked loop in the second slave electronics module to thephase-locked loop in the first slave electronics module.
 15. A seismicdata acquisition system, comprising first and second pluralities ofseismic sensors sending seismic signals to the first and second slaveelectronics modules, and using the seismic telemetry interface of claim11.
 16. A seismic telemetry interface, comprising: a master electronicsmodule including a master clock; a slave electronics module including aphase-locked loop; a two-conductor line connecting the masterelectronics module to the slave electronics module; and wherein themaster electronics module transmits a portion of a digital command tothe slave electronics module over the two-conductor line at a first timeand the slave electronics module transmits a portion of a digital datasequence to the master electronics module over the two-conductor line ata different second time.
 17. A seismic telemetry interface as in claim16 wherein the master electronics module transmits consecutive portionsof a digital command at fixed intervals.
 18. A seismic telemetryinterface as in claim 17 wherein the consecutive portions of a digitalcommand are encoded to provide digital level transitions defining syncpoints occurring at the fixed intervals.
 19. A seismic telemetryinterface as in claim 16 wherein the master electronics module transmitsportions of digital commands at fixed intervals defining contiguousframes, the first time marking the start of a frame and the second timeoccurring later in the frame.
 20. A seismic telemetry interface as inclaim 16 wherein the master clock in the master electronics moduleincludes a phase-locked loop.
 21. A seismic telemetry interface as inclaim 16 wherein the slave electronics module derives timing informationfrom the digital command to lock the phase-locked loop in the slaveelectronics module to the master clock.
 22. A seismic telemetryinterface as in claim 16 wherein the digital command is encoded withdigital level transitions spaced at fixed intervals to provide thetiming information to lock the phase-locked loop in the slaveelectronics module.
 23. A seismic telemetry interface as in claim 16wherein the master electronics module further comprises a power supplycoupled to the two-conductor line to supply power to the slaveelectronics module.
 24. A seismic data acquisition system, comprising aplurality of seismic sensors sending seismic signals to the slaveelectronics module and using the seismic telemetry interface of claim16.